1. Technical Field
The present invention relates to a method of manufacturing a molding die that is used in molding expandable resin particles of a thermoplastic resin into an expanded molding and is capable of forming a protrusion and recess pattern on a surface of the expanded molding, as well as to an expanded molding.
2. Background Art
An expanded molding of a thermoplastic resin such as polystyrene, polyethylene or polypropylene is manufactured by means of a molding die that includes a first molding die of a fixed side and a second molding die that is movable in a die closing and opening direction relative to the first molding die, in which the molding die forms a molding cavity matched in shape to a target expanded molding when it is closed, by filling expandable resin particles of a thermoplastic resin in the molding cavity, then heating the expandable resin particles within the molding cavity by heating medium such as steam, thereby expanding the expandable resin particles and fusing the same together.
However, the above method has a problem in that a honeycomb pattern of the expandable resin particles is formed on the surface of the expanded molding that has been manufactured by the expansion and fusing of the expandable resin particles, and this deteriorates an appearance of the expanded molding.
Therefore, a molding die having a protrusion and recess pattern is broadly used. This molding die has a protrusion and recess pattern preformed on its molding surface so that this protrusion and recess pattern is transferred onto the surface of an expanded molding, thereby making the honeycomb pattern of the expandable resin particles insignificant and thus enabling manufacturing of an expanded molding having a good appearance.
Conventionally, as a known method of manufacturing a molding die having a protrusion and recess pattern, for example Japanese Utility Model Registration No. 3045015 describes a method of manufacturing a molding die, which includes first fixing a wire mesh or punching metal onto a surface of a wooden mold (master mold), fabricating a sand mold having a protrusion and recess pattern transferred from the wire mesh or punching metal by using the wooden mold, and then casting a molding die having a protrusion and recess pattern transferred from the sand mold by using the sand mold.
However, the method involving fixing the wire mesh onto the wooden mold has a problem in that, due to the woven structure of the wire mesh having wires of the mesh woven to be overlapped and intersected with each other, gaps are caused between the wooden mold and the wire mesh by the overlapped and intersected wires, and when a sand mold is fabricated by using this wooden mold, molding sands invade into the inside of the wire mesh through the gaps between the wooden mold and the wire mesh, thereby causing a trouble, such as break of the sand mold when the wooden mold is pulled out.
According to the method involving fixing the punching metal onto the wooden mold, it is difficult to fix the punching metal to the wooden mold while bringing the punching metal completely into conformity with a curved molding surface or angular recess of the wooden mold, since the punching metal is rigid and has little flexibility. Thus, there is a problem in that the punching metal can be used only for a wooden mold having a relatively simple shape with no curved surface or angular recess.    Patent Document 1: Japanese Utility Model Registration No. 3045015